1. Field of the Invention
The present invention generally relates to photolithography and associated methods for exposing semiconductor substrates.
2. Description of Related Art
Lithographic exposure apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device may generate a circuit pattern corresponding to an individual layer of the IC. In photolithography, a beam of radiation is patterned by having that beam traverse the patterning device, and is projected by a projection system of the lithographic apparatus onto a target portion (e.g., comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of photo-activated resist (i.e., photoresist) material, such as to image the desired pattern in the resist. In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time.
In the semiconductor industry, the continual demand for smaller semiconductor devices, having smaller patterns and features on the wafer substrate, is pushing the limits on the optical resolution that can be achieved by lithographic exposure apparatus.
Generally, the smallest size of a repeatable feature (e.g., “half-pitch”) of a pattern exposed on the wafer substrate that can be optically resolved by lithographic exposure apparatus, depends on attributes of the projection system and the (patterned) projection beam of radiation. In particular, the optical resolution for half-pitch feature size may be derived by using the simplified form of the Rayleigh resolution equation:p0.5.=k1·λ/NA, with k1≧0.25  (1)
where: p0.5 represents the repeatable feature size (e.g., “half-pitch”) in nm;
NA represents the numerical aperture of projection system;
λ represents the wavelength of projection beam, in nm; and
k1, is a factor representative for the achievable optical resolution limit for the half-pitch feature size.
As indicated above, the theoretical optical resolution half-pitch lower limit for k1, is 0.25. In an attempt to approach the k1=0.25 barrier, considerable efforts have been directed to develop expensive technologies that are capable of employing shorter wavelengths and/or higher numerical apertures, thus allowing production of smaller features approaching the k1≧0.25 constraint.
Fabrication of an integrated circuit pattern involves the control of space tolerances between features, as well as control of feature dimension tolerances. In particular the control of tolerances of the smallest dimensions such as for example the sizes of contacts or the width of lines or of spaces between lines permitted in the fabrication of the integrated circuit device is of importance. The size of these most critical dimensions is referred to as the critical dimension (“CD”). Features comprising a minimum size substantially equal to the CD are referred to as “CD-sized features” in the present text.
Further, a variety of phenomena that accompany low k1 imaging, such as line-end retraction, corner rounding, variation of CD versus pitch, mask error factor (“MEF”), and line-edge roughness (“LER”) may lead to a loss of feature fidelity beyond tolerance. In particular, MEF contributes to variations in lengths of polysilicon gates, reducing performance of the integrated circuit. The MEF is defined as the ratio of a change in CD of CD-sized features in resist in response to a change of the corresponding size of the corresponding features on the mask, whereby the latter size is normalized to substrate level taking into account the demagnification of the projection system. In the field of photolithography, Mask Error Factor is, alternatively, also referred to as Mask Error Enhancement Factor (“MEEF”). The two concepts are identical, and in the present text referred to as Mask Error Factor or MEF.
Lithographic apparatus as mentioned above typically include a radiation system and a projection system. The radiation system generally includes an illumination system. The illumination system receives radiation from a source, such as a laser, and produces an illumination beam for illuminating an object, such as the patterning device (e.g., a mask on a mask table). Within a typical illumination system, the beam is shaped and controlled such that at a pupil plane of the illumination system the beam has a desired spatial intensity distribution. Such a spatial intensity distribution at the pupil plane effectively acts as a virtual radiation source for producing the illumination beam. Various shapes of said intensity distribution, consisting of (substantially uniform) light areas on a dark background, can be used. Any such shape will be referred to hereinafter as an “illumination mode”. Known illumination modes include: conventional (a top-hat shaped intensity distribution in said pupil), annular, dipole, quadrupole and more complex shaped arrangements of the illumination pupil intensity distribution. A lateral position in said pupil plane corresponds to an angle of incidence at the patterning device, and any such angle of incidence is commonly expressed as a fraction sigma (σ) of a numerical aperture NA of the projection system. Therefore, a more complete characterization of the intensity distribution in a pupil of the illumination system involves, besides an indication of the illumination mode, also an indication of parameters of the illumination mode, such as, for example, σ and NA. A combination of an illumination mode and corresponding parameters of said illumination mode is referred to hereinafter as an “illumination setting”. Known illumination settings include: a “conventional” illumination setting (whereby the intensity distribution in an illumination pupil is substantially uniform up to a certain radius defined by a parameter value of σ, where 0<σ<1, and a parameter value of the numerical aperture NA of the projection system), an annular setting, a dipole setting, a quadrupole setting and more complex arrangements. An annular or a multipole setting are typically characterized by the parameters σinner and σouter respectively indicating an inner and outer radial extent of an annulus or of poles. Such illumination modes are providing off-axis illumination of a patterning device. Illumination settings may be formed in various ways. The σ value of a conventional illumination mode can be controlled using a zoom lens while σinner and σouter values of an annular mode can be controlled using a zoom-axicon. The NA value can be controlled using a settable iris diaphragm in the projection system.
More complex settings (such as said dipole and quadrupole modes) may be formed using a diaphragm with appropriate apertures in the pupil plane or by a diffractive optical element. Typically, said diffractive optical element is arranged to generate a preselected angular intensity distribution upstream of a pupil plane of the illumination system. This angular intensity distribution is transformed into a corresponding spatial intensity distribution in the pupil plane of the illumination system.
In particular at high numerical aperture (NA>0.85) and using off axis illumination modes for illumination the patterning device, MEF and LER are most prominent errors limiting a further reduction of k1 for those lithographic processes whereby the task is to print in positive tone resist semi-dense regularly spaced patterns of trenches or lines, the width sized at critical dimension and the features spaced apart about three times the critical dimension.